Electro-optical displays are well known in the art. Such displays typically feature an electro-optical material enclosed by a hermetic seal within a cavity between two plates of the display of which either one or both plates are made from a transparent material such as glass or quartz. The electro-optical material is generally of the class of materials whose ability to block or transmit light is dependent upon the direction in which light impinges upon its molecular structure with its ability to transmit or block light dependent upon whether it is an electrically energized or an electrically un-energized state whereby it is able to re-orient the direction of its molecular structure with respect to the direction of the incident light. The means by which the electro-optical material is electrically energized is commonly provided by coating the inside surface of the plates adjacent the electro-optical material with a transparent electrically conductive material such as tin oxide or indium oxide. The images transmitted to a viewer of the display are provided by forming the conductive coating into one or more discrete configurations on at least one of the plates so that only the electro-optical material between the configuration and the coating on the opposite plate is electrically energized when electrical power is connected to the particular configuration.
Electro-optical materials suitable for use in electro-optical displays are well known to those ordinarily skilled in the art and have been the object of considerable study and development for many years. Generally, they comprise a unique class of organic materials having a crystalline structure which is able to be rotated or otherwise re-oriented by an electric field and, as a result of such re-orientation, effect the amount of light that is able to be transmitted through the material. Electro-optical materials in common use today are generally known as "liquid crystalline" materials. Liquid crystalline materials are classified according to their liquid crystal packing structure into smectic, chlosteric and nematic type materials. Generally, smectic type liquid crystals feature a parallel layered dispersion of the organic crystalline structure in an amorphic organic fluid medium whereas the chlosteric type features an organic crystalline structure that is in the form of coils and the nematic type features a uniformly disposed organic crystalline structure within an amorphic organic medium.
Most commonly used today for electro-optical displays are nematic type liquid crystals whose polarity can be controlled by means of the location and type of chemical groups that are attached to the organic crystalline structure. By controlling polarity, nematic liquid crystalline materials have developed into those having positive dielectric anistrophy and those having negative dielectric anistrophy. Those having positive dielectric anistrophy tend to align parallel to the direction of an electric field and those having negative dielectric anistrophy tend to align at 90.degree. to the direction of an electric field imposed across the material Although nematic liquid crystalline materials having positive dielectric anistrophy are most popular today for use in electro-optical displays, the invention contemplates electro-optical displays using any suitable electro-optical material or mixtures of such materials with or without additional materials such as dichroic organic dyes, colorants and homologous non-liquid crystalline materials and the like.
The electrically conductive coatings on the side of the plates facing the electro-optical material may be electrically energized by imposing either a direct current or alternating current voltage between the coatings. Commonly, the voltage is of a positive polarity derived by trimming the negative polarity from an alternating current source such that a pulsed current of positive polarity is provided. For electro-optical displays utilizing twisted nematic liquid crystals, the voltage is commonly from about 3 to about 10 volts.
Although the voltage may be imposed across the electro-optical material by attaching one of the conductive coatings to the ground side of the voltage source and the other conductive coating to the active side of the voltage source, it is preferred to attach only the active side of the voltage source to a particular separate conductive lead of one of the coatings of negligble resistance for transmission of the current to an electrical connecting member of controlled conductivity that extends between and electrically connects the conductive lead to a conductive lead of the conductive coating on the other plate. Although the electrical connecting member may have any suitable shape, it preferably has a cylindrical shape having its opposite ends abutting respectively against the conductive coating leads on the opposed spaced-apart plates of the display. The conductive connecting member typically has an electrical resistance controlled from about 1 ohm to about 10 ohms and is preferably isolated from the electro-optical material in order to prevent any adverse effect of one upon the other. Typically, the conductive connecting member extends between the plates either outside of the cavity or through the seal enclosing the electro-optical material as a means of insuring isolation between the conductive connecting member and the electro-optical material.
An example of an electro-optical display utilziling an electrically conductive epoxy material as a conductive connector between the facing conductive coatings of the spaced apart plates of the display is disclosed in U.S. Pat. No. 3,881,809. The connector extends through the gasket enclosing the liquid crystalline material of the display and electrically connects the conductive coating on one of the plates to the conductive coating on the other plate of the display.
Commonly, the highly conductive connecting member is provided by blending high amounts of a highly electrically conductive material such as silver into suitable non-electrically conductive resins such as the epoxy resins disclosed in U.S. Pat. No. 3,881,809. An example of the use of a solderable metal coating in a hole in the plates, of a liquid crystal cell for providing an electrical bridge between the plates is disclosed in U.S. Pat. No. 4,106,860.
It has been found, however, that electro-optical displays using conductive connecting members of the above type are not entirely satisfactory in that they are not able to maintain continuous electrical contact between the conductive leads on the plates of the display when the display is subjected to temperature cycling due generally to either the inability of the connecting member material to withstand the stress arising from the changes in temperature or the inabiliy of the connecting member material to maintain a bond to the conductive coating on both of the plates over the range of temperature cycling.
In view of the above and in order to provide electro-optical displays that are operable over a broad temperature range, a need exists to provide a material suitable for use as an electrically conductive connecting member for providing an electrical bridge between the respective conductive coating leads disposed on the plates of the display that has sufficient resiliency and is able to bond to and to maintain the bond to the respective plates and conductive coatings over a broad temperature cycling range. Likewise, it is also of advantage to use such material at other locations associated with electro-optical displays where the maintenance of electrical continuity over a broad temperature cycling range is desirable.